In recent years, a technique of constructing a thin film transistor (TFT) with a semiconductor thin film (with a thickness of approximately several to several hundreds nm) formed over a substrate having an insulating surface has attracted attention. The thin film transistor is widely used in an electronic device such as an IC or an electro-optical device, and is rushed to be developed especially as a switching element of an image display device.
Various applications using such the image display device have been expected. Especially, an application of a portable device has attracted attention. Currently, a glass substrate or a quartz substrate is often used; however, these substrates have defects of being fragile and heavy. The size of a glass substrate or a quartz substrate is difficult to be increased, and so these substrates are unsuitable for mass-production. Consequently, a TFT element is attempted to be formed over a substrate having flexibility, typically, a flexible plastic substrate.
However, the present situation is that maximum temperature of a process should be lowered since a plastic film has low heat resistance, as a result, a TFT having electric characteristics as favorable as those of the TFT formed over a glass substrate cannot be formed. Therefore, a high performance liquid crystal display device or light-emitting element using a plastic film has not been realized.
A technique of peeling an element formed over a glass substrate and transferring the peeled element to another base material, for example, a plastic film has been proposed.
This applicant proposes peeling and transferring techniques disclosed in patent documents 1 and 2. Further, this applicant proposes peeling and transferring techniques disclosed in a patent document 6.
In a minute device such as an IC, a semiconductor wafer pasted onto a pressure-sensitive adhesive sheet is cut into each chip and the cut semiconductor element is picked up from the pressure-sensitive adhesive sheet to be mounted to a circuit substrate which constructs an IC card or the like. However, the semiconductor element has defects of being easily damaged and fragile since a semiconductor wafer is used.
Further, a method for peeling a layer to be peeled formed over a substrate via a separating layer from the substrate has been already proposed. For example, patent documents 3 and 4 disclose the technique that a separating layer made from amorphous silicon (or polysilicon) is provided, hydrogen is released included in the amorphous silicon by emitting laser light through a substrate to produce space, and the substrate is separated from the separating layer. In addition, patent document 5 discloses that a liquid crystal display device is completed by pasting the layer to be peeled (which is referred to as a layer to be transferred) onto a plastic film by using the foregoing technique.
In the foregoing method, however, a substrate having a high light-transmitting property is used and energy which is sufficient to pass through the substrate to release hydrogen included in amorphous silicon is given, and so a whole surface is desirably irradiated with comparative large laser light. Accordingly, there arises a problem that the laser light damages the layer to be peeled. In the case that an element is manufactured over the separating layer in the foregoing method, hydrogen included in the separating layer is released to be reduced when heat treatment at high temperature is performed in a manufacturing process of the element. Subsequently, there is a threat that peeling cannot be sufficiently carried out even if laser light is emitted to the separating layer. Therefore, there is a problem that processes after forming the separating layer are limited in order to keep an amount of hydrogen included in the separating layer. In the foregoing patent publication, there is description that a light-shielding layer or a reflection layer is provided in order to prevent damages of a layer to be peeled, in that case, however, a transparent liquid crystal display device is difficult to be manufactured. In addition, it is difficult to peel a large layer to be peeled by the foregoing method.
Patent document 1: Unexamined patent publication No. 8-288522
Patent document 2: Unexamined patent publication No. 8-250745
Patent document 3: Unexamined patent publication No. 10-125929
Patent document 4: Unexamined patent publication No. 10-125931
Patent document 5: Unexamined patent publication No. 10-125930
Patent document 6: Unexamined patent publication No. 2003-174153